1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for driving a liquid crystal display device. In particular, the present invention relates to a circuit and a method for driving a matrix-type liquid crystal display device capable of conducting a gray-scale display for use in various office automation apparatuses such as personal computers, word processors and the like, multimedia terminals, video game machines, audio visual apparatuses, etc.
2. Description of the Related Art
Conventionally, a line-sequential scanning method has been employed for driving a simple matrix type liquid crystal display device which employs a liquid crystal material responsive to an effective voltage such as a TN (Twisted Nematic) liquid crystal material or an STN (Super Twisted Nematic) liquid crystal material. According to this method, scanning signals are successively applied to row electrodes as scanning lines so that one row electrode is selected at a time. In synchronization with this selection of one row electrode, signals according to image data for pixels on the selected row electrode are applied to column electrodes as data lines.
Recently, with a growing trend toward multimedia apparatuses, fast-responding STN liquid crystal materials have been improved. Along with this development, it has become possible to realize a motion picture display using the STN liquid crystal material, and color liquid crystal displays have been realized. With these achievements, there is a growing demand for realizing a multi-color display with the STN liquid crystal material for displaying Television images, images for amusement purposes, and the like.
However, in a fast-responding liquid crystal display device utilizing the conventional line-sequential driving method, as the number of scanning lines of the liquid crystal panel increases, the frame response effect becomes greater, thereby lowering the contrast of the display. A way to reduce this deterioration of display quality is to drive a liquid crystal display device with a higher frame frequency. Recently, there have been proposed other driving methods such as the following two methods which can reduce the frame response effect more effectively.
One of the two methods is to select all the row electrodes included in the display panel simultaneously. This is called an active addressing method (see T. J. Scheffer et al.: "Active Addressing Method for High-Contrast Video-Rate STN Displays", SID '92 DIGEST, pages 228 to 231).
The other method is to divide the row electrodes included in the display panel into blocks and select a block of row electrodes at a time. This is called a multiple line selection method (see T. N. Ruckmongathan et al.: "A New Addressing Technique for Fast Responding STN LCDS", JAPAN DISPLAY '92, p65).
The basic display principle of these two methods is to perform an orthogonal transformation for image data based on an orthogonal matrix such as Hadamard's matrix or Walsh's matrix, thereafter performing an inverse transformation for the transformed image data on a liquid crystal panel. Driving signals have waveforms such that some or all of the row electrodes are selected simultaneously in a single-frame period. These two driving methods utilize the cumulative response effect of the liquid crystal material, where a plurality of relatively small scanning selection pulses are applied to a row electrode instead of a single large pulse in a single-frame period, thereby maintaining both the high response rate and the high contrast of the display.
As a method for conducting a gray-scale display with a display device based on the line-sequential driving method, a frame modulation method or a pulse width modulation method is widely employed. In these methods, the amplitude of driving voltages is fixed while the duration of voltage application is varied.
According to the frame modulation method, either one of fixed voltages (ON and OFF display voltages) is selectively applied to a pixel for each frame according to the gray-scale level to be effected on the pixel for the frame. Thus, more than one gray-scale levels are obtained for each pixel as an average state over a plurality of frames. The gray-scale level of a pixel is based on the number of frames during which the ON display voltage is applied to the pixel among the averaging frames.
According to the pulse width modulation method, the amplitude of applied voltages is also fixed (i.e., the ON and OFF display voltages are fixed). However, the pulse width of a signal to be applied to each pixel is modulated based on the gray-scale level to be effected on the pixel so as to obtain a plurality of levels of the gray-scale display.
The frame modulation method or the pulse width modulation method may be employed for a display device using the multiple line selection method or the active addressing method, as well as for the display device using the line-sequential driving method. However, there has also been proposed an amplitude modulation method as a new gray-scale display method for the multiple line selection display devices or the active addressing display devices. According to the amplitude modulation method, the amplitude of an applied voltage is modulated while the duration of voltage application is fixed, so that a gray-scale display with more than one levels is conducted. This method is described in, for example, Japanese Laid-Open Patent Publication No. 6-89082 and Japanese Laid-Open Patent Publication No. 6-138854.
These conventional gray-scale display methods have disadvantages as follows. First, regarding the frame modulation method, in order to effect a certain number of gray-scale levels with this method, a number (the number of gray-scale levels -1) of frames are required. Therefore, as the number of gray-scale levels increases, the number of frames used to effect gray-scale display increases, whereby flickers or wavings in the displayed images may become visible. Moreover, such an undesirable phenomenon becomes more conspicuous when this modulation method is employed in a fast-responding liquid crystal panel.
Next, regarding the pulse width modulation method, in order to effect a certain number of gray-scale levels with this method, the ratio of the minimum and maximum pulse widths must be set to the number of grayscale levels. Accordingly, as the number of gray-scale levels increases, the minimum pulse width decreases. Moreover, as the liquid crystal panel becomes larger, the electrode resistance increases. Therefore, particularly when conducting a gray-scale display on a large liquid crystal panel, waveform distortion of a driving voltage signal becomes large at locations remote from the driving point due to the reduced pulse width and the increased resistance. This allows the non-uniformity of the display to occur more easily. Regarding the amplitude modulation method, in order to obtain voltage amplitudes corresponding to the gray-scale display data, the method requires a complicated large-scale arithmetic circuit for performing square-sum calculation and square-root extraction, and a high-precision liquid crystal driver which outputs a signal having the analog voltage amplitude. These additional circuits result in a large-scale circuit in the display device, and increases the amount of power consumption and the manufacturing cost of the device.